Plasma-sprayed Zirconia Coatings Research Articles

Effects of Splat Interfaces, Monoclinic Phase and Grain Boundaries on the Thermal Conductivity of Plasma Sprayed Yttria-Stabilized Zirconia Coatings

Microstructure has a significant influence on the thermal conductivity of thermal barrier coating (TBC) systems. In this work, the microstructures including splat interface, monoclinic phase and grain boundaries in the YSZ air plasma spraying (APS) TBC systems are investigated. A finite element simulation model based on electron backscatter diffraction (EBSD) images is established. It is found that the simulation results of thermal conductivity are in good agreement with the experimental results. Using this model, the effect coefficient of splat interface, monoclinic phase and grain boundaries on thermal conductivity are calculated. Results show that the splat interface influences the thermal conductivity of the TBCs. Those results provide important guidance for reducing the thermal conductivity of thermal barrier coatings.

Effect of electron beam remelting treatments on the performances of plasma sprayed zirconia coatings

In this study, zirconia coatings were deposited on stainless steel using air plasma spraying (APS), and electron beam remelting treatments (EBRTs) were used to improve the performances of the plasma sprayed coatings. The properties of the zirconia coatings such as porosity, microhardness, surface roughness, phase transformation and wear behaviors were characterized and measured. The experimental results showed that electron beam treatments of the plasma sprayed coatings generated a compact remelted layer on top of the surface composed of columnar structure. EBRTs significantly reduced the porosity and surface roughness. The surface microhardness had also nearly doubled after EBRTs. Compared with the as-sprayed zirconia coating, wear behaviors of the electron beam treated coatings had considerably improved.

The Influence of Heat Treatments on the Porosity of Suspension Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Suspension plasma-sprayed coatings are produced using fine-grained feedstock. This allows to control the porosity and to achieve low thermal conductivity which makes the coatings attractive as topcoats in thermal barrier coatings (TBCs). Used in gas turbine applications, TBCs are exposed to high temperature exhaust gases which lead to microstructure alterations. In order to obtain coatings with optimized thermomechanical properties, microstructure alterations like closing of pores and opening of cracks have to be taken into account. Hence, in this study, TBC topcoats consisting of 4 mol.% yttria-stabilized zirconia were heat-treated in air at 1150 °C and thereafter the coating porosity was investigated using image analysis (IA) and nuclear magnetic resonance (NMR) cryoporometry. Both IA and NMR cryoporometry showed that the porosity changed as a result of the heat treatment for all investigated coatings. In fact, both techniques showed that the fine porosity decreased as a result of the heat treatment, while IA also showed an increase in the coarse porosity. When studying the coatings using scanning electron microscopy, it was noticed that finer pores and cracks disappeared and larger pores grew slightly and achieved a more distinct shape as the material seemed to become more compact.

The microstructural studies of suspension plasma sprayed zirconia coatings with the use of high-energy plasma torches

The presented studies are focused on the microstructure characterization of zirconia-based coatings deposited by two types of high-energy plasma torches: (i) Axial III; and, (ii) hybrid version of Water-Stabilized Plasma (WSP) torch. The suspensions were formulated using solid dispersed phase of: (i) zirconia stabilized with 14wt% of Y2O3 and (ii) zirconia stabilized with 24wt% of CeO2+2.5wt% of Y2O3 and continuous phase of water with ethanol. The spray process parameters were optimized for each plasma set-up individually. The in-flight observations (shadowgraphy) were performed to optimize the injection of the liquid feedstock into the plasma jet. Then the coating's morphology and coating/substrate interface were characterized using conventional light microscopy and scanning electron microscopy (SEM). The results showed that through the change of deposition parameters various coatings microstructures could be obtained, in particular columnar and two-zones structures. The EDS/EDX and XRD studies showed that there was no significant change in chemical/phase composition of zirconia material before and after spraying. Electron backscatter diffraction (EBSD) method allowed to analyze the grain size in the coating microstructure as well as crystallographic orientation of individual grains. The results showed that coatings were characterized by submicrometric microstructure what corresponded to the size of powder particles used to formulate suspension. No texture was observed in the coatings microstructure. The surface topography analysis which was performed by confocal scanning laser microscopy (CSLM) and Shape From Shading (SFS) technique proved the great influence of suspension concentration on the coating structure.

HOT CORROSION OF YTTRIUM STABILIZED ZIRCONIA COATINGS DEPOSITED BY AIR PLASMA SPRAY ON A NICKEL-BASED SUPERALLOY

In this research, the electrochemical impedance spectroscopy (EIS) and Tafel analysis were utilized to study the hot corrosion performance at 700[Formula: see text]C of air plasma-sprayed (APS) yttria-stabilized zirconia (YSZ) coatings with a NiCrAlY bond coat grown by high velocity oxygen fuel spraying (HVOF), deposited on an INCONEL 625 substrate, in contact with corrosive solids salts as vanadium pentoxide V2O5 and sodium sulfate Na2SO4. The EIS data were interpreted based on proposed equivalent electrical circuits using a suitable fitting procedure performed with Echem AnalystTM Software. Phase transformations and microstructural development were examined using X-ray diffraction (XRD), with Rietveld refinement for quantitative phase analysis, scanning electron microscopy (SEM) was used to determinate the coating morphology and corrosion products. The XRD analysis indicated that the reaction between sodium vanadate (NaVO[Formula: see text] and yttrium oxide (Y2O[Formula: see text] produces yttrium vanadate (YVO[Formula: see text] and leads to the transformation from tetragonal to monoclinic zirconia phase.

Advanced Microscopic Study of Suspension Plasma-Sprayed Zirconia Coatings with Different Microstructures

The present paper is focused on the characterization of the differences between two microstructures that can be obtained using SPS technology, namely (i) columnar and (ii) two-zone microstructure including lamellas and fine unmelted particulates. The optimization of spray parameters was made, and the advanced microstructural studies of obtained coatings were performed. The work was focused on zirconia stabilized by yttria (YSZ, ZrO2 + 14 wt.% Y2O3) and both by yttria and ceria (YCSZ, ZrO2 + 24 wt.% CeO2 + 2.5 wt.% Y2O3) which are frequently used as thermal barrier coatings. Two types of microstructure were achieved using two different plasma torches, namely SG-100 of Praxair and Triplex of Oerlikon Metco. The microstructure of prepared coatings was analyzed using scanning electron microscopy with secondary electrons detector and backscattered electrons. Energy dispersive spectroscopy was performed to analyze the chemical composition of sprayed coatings. By electron backscatter diffraction grain shape, size, and crystal orientation were determined. The analysis enabled the discussion of the coatings growth mechanism. Finally, the Shape From Shading technique was applied to recreate and to analyze 3D views of coatings’ topographies, and using laser confocal microscopy, the surface roughness was examined.

Numerical investigation on the effective thermal conductivity of plasma sprayed zirconia coatings

Due to the complex microstructure of the plasma sprayed yttria-stabilized zirconia (YSZ) coatings, there is still no good numerical method to establish the relationship between the effective thermal conductivity and the microstructure. In this paper, two different approaches have been developed to investigate the effective thermal conductivity of the thermal barrier coatings (TBCs), which are both based on the Lattice Boltzmann method (LBM). One approach employs the quartet structure generation set (QSGS) method to construct the isotropic or anisotropic microstructure without experimental techniques. In this method, through changing the parameters of QSGS algorithm, it can estimate the effective thermal conductivity of TBCs fabricated by different plasma spray technologies. The other method is related to the real microstructure of TBCs, which employs the image analysis software to convert the real experimental micrograph obtained using a scanning electron microscope (SEM) into the binary image. Although the first method is more available and practical without analyzing the microstructural image, the second method can estimate the effective thermal conductivity for the plasma sprayed coatings more precisely. However, comparing the predicted results and the experimental data, there are some limitations in both methods. Additionally, the effective thermal conductivity of gradient TBCs is also analyzed in this paper. The results will provide a useful guide for predicting the effective thermal conductivity of plasma sprayed coatings and a strong foundation for designing the gas turbine engines.

Effect of Dimensions of Non-bonded Lamellar Interfaces on the Stress Distribution in APS-TBCs

Cracking behavior has been considered for the top coating in the thermal barrier coating system. The effect of the dimension and position of non-bonded lamellar interfaces on the stress distribution is investigated by the simulation method. Plasma-sprayed yttria-stabilized zirconia coatings are deposited at different spraying distances. The crack propagation behavior is studied by statistical analysis after the coatings have experienced different thermal cycles. The simulation results reveal that the normal stresses at the tips of non-bonded lamellar interfaces increase with the increment of the length of interfaces and with the decrease of the distance between them. The stress level almost stays the same with the changing of lamellar thickness. Experimental results indicate the correctness of the simulation. It is found that the length of non-bonded lamellar interfaces in the as-deposited coatings shows an increasing tendency with the spraying distance. The total crack length of coatings increases with the number of thermal cycles and the spraying distance after thermal cycling tests.

The key process parameters influencing formation of columnar microstructure in suspension plasma sprayed zirconia coatings

The paper discusses the screening of experimental variables leading to formation of a columnar microstructure in suspension plasma sprayed zirconia coatings. These variables tested in 12 experimental runs included: (i) 2 types of zirconia powder; (ii) 4 concentration of solids in suspensions; (iii) 4 substrate preparation methods; and (iv) 2 plasma spray setups. Two different, commercially available, powders were used to formulate the suspensions. Yttria and ceria stabilized zirconia of composition ZrO2+24wt.% CeO2+2.5wt.% Y2O3 (YCSZ) was milled the decrease the particles sizes. The yttria stabilized zirconia of composition ZrO2+14wt.% Y2O3 (14YSZ) was used as received. The coatings were deposited on 304L stainless steel substrates which had the surface prepared by: (i) grid blasting; (ii) grinding; (iii) turning; and (iv) laser treatment. The 3D topographies of substrates' surfaces were characterized and their roughnesses were measured. The suspensions were plasma sprayed using the following plasma torches: SG-100 of Praixair and Triplex of Sulzer-Metco. The microstructure of powders and coatings was analyzed by optical microscopy, scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) as well as by X-ray diffraction. The columnar microstructure was formed in coatings sprayed with both plasma setups sprayed using finer 14YSZ powder suspensions. The substrate surface preparation as well as low concentration of solids in suspension promoted their formation. Rietveld method was applied to determine the quantity of different phases in the structure of coatings and to calculate the lattice parameters. The YCSZ coatings crystallized in mainly tetragonal phase with a small content of monoclinic phase. The 14YSZ crystallized in cubic phase. Finally, the thermal diffusivity of coatings was characterized up to 523K with the use of laser flash method and thermal conductivities of coatings were determined. The conductivities were in the range from 0.6 to 1.1W/(mK) depending on temperature for YCSZ and 14YSZ coatings.

Abrasive Wear Analysis of Plasma-Sprayed LaCeYSZ Nanocomposite Coatings Using Experimental Design and ANN

The abrasive wear characteristics of plasma-sprayed nanostructured yttria-stabilized zirconia (YSZ) coatings on Inconel 718 substrates was evaluated using AFS 50/70-grade silica sand as abrasives. This article depicts the dependence of abrasive wear characteristics of plasma-sprayed nanocomposite LaCeYSZ coatings on abrading distance, keeping the applied load constant. The influence of four operating parameters—that is, load, wheel speed, time, and temperature with four different levels each—on the performance output (i.e., abrasion wear rate) is studied using Taguchi's L16 orthogonal array design and analysis of variance (ANOVA). Out of the four parameters, load has been found to be most significant factor followed, by speed of the abrasive wheel and temperature influencing abrasion. The morphology of the worn-out surface also showed microcutting and small crater formation in the binder matrix caused by the repetitive impacts of abrasive particles. It was observed that coating with nano-LaCeYSZ grains exhibited higher wear resistance compared to conventional YSZ coating and the reason may be attributed to embedded crack-arresting nanozones, which toughen the coating. An artificial neural network (ANN) approach is then implemented taking into account training and test procedures to predict the triboperformance under different operating conditions. This technique helps in saving time and resources for a large number of experimental trials and successfully predicts the wear rate of the coatings both within and beyond the experimental domain.

522 プラズマ溶射ジルコニアコーティングの熱伝導率に及ぼす組織の影響

522 プラズマ溶射ジルコニアコーティングの熱伝導率に及ぼす組織の影響

Microstructural dependency of thermal expansion and sintering shrinkage in plasma-sprayed zirconia coatings

The effect of microstructure upon thermal expansion and sintering shrinkage in plasma-sprayed zirconia coatings was investigated by an accurate dilatometry. Cut-out samples with different microstructures were prepared from 5.8-mm-thick atmospheric plasma-sprayed (APS) and water-stabilized plasma coatings (WSP). It was quantitatively determined that the samples cut out of different thickness positions had minor differences in microstructure, and these APS samples largely differed from the WSP samples. The thermal expansion behaviors of all the samples coincided after a short annealing time despite their initial structural differences. On the other hand, all the samples showed a significant difference in sintering shrinkages with annealing at 1400°C. This result was consistent with the theoretical result calculated with Cipitria's sintering model in terms of the relationship between shrinkage and microstructure. It was therefore demonstrated that the initial microstructure, particularly the splat thickness and the inter-splat pore height, exerts a great influence on the sinterability of plasma-sprayed coatings.

Finite Element Study of Influence Factors on Thermal Conductivity of Plasma Sprayed Zirconia Coatings Based on Digital Image Processing

Combining digital image processing with finite element mesh generation,finite element models based on actual microstructures of plasma sprayed zirconia coatings were built.By employing finite element method in conjunction with Fourier’s equation,the apparent thermal conductivity of zirconia coatings affected by defects,in-cluding pores and cracks were calculated.For comparison,the coating’s thermal conductivity was measured by the la-ser pulse method.The difference between values of the finite element method and experimental results are told to ana-lyze effects of defects and splat interfaces on the thermal conductivity.The results show that splat interfaces are as equally important as defects in defining the thermal conductivity of plasma sprayed coatings in the spray direction.

Characterization of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings by Cathodoluminescence

The occurrence of monoclinic zirconia phase has an important impact on the performance of thermal barrier coatings (TBC) of yttria-stabilized zirconia (YSZ). Therefore, a reliable method is needed to detect its contents and to investigate also its spatial distribution within the parent microstructure. This was the motivation to apply cathodoluminescence (CL) spectroscopy. YSZ coatings with different porosities were manufactured by atmospheric plasma spraying. CL analysis yielded monoclinic phase contents of 5.2 ± 1.6% for the high-porous sample and 3.4 ± 0.5% for the low-porous sample. The results were qualitatively confirmed by x-ray diffraction (XRD). However, due to its lower detection sensitivity the XRD results are quantitatively on lower level. Owing to its synthesis method, the applied powder feedstock showed a considerable content of monoclinic phase. The lower the particle temperatures were the larger fraction of monoclinic phase remained untransformed. This has to be considered when spraying high-porous TBCs.

Characterization of ceria–yttria stabilized zirconia plasma-sprayed coatings

Ceria–yttria stabilized zirconia (CYSZ) coatings were prepared by air plasma-sprayed on the nickel alloy. The as-sprayed CYSZ coatings and heat-treated CYSZ coatings were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The XPS data indicated the coexistence of Ce 3+, Ce 4+, Y 3+ and Zr 4+ ions near the surface of the as-sprayed CYSZ coatings and the disappearance of Ce 3+ ions in the CYSZ coatings after thermal treatment at 1000 °C for 15 h. From the XRD patterns, the solid solution of CeO 2–Y 2O 3–ZrO 2 formed in the CYSZ coatings because of the lack of any features from Y 2O 3 and ZrO 2 single phases. After thermal treatment, the main phases of all the samples were consistent with the characteristic peaks of cubic ZrO 2.

Nano-phenomena during exposure of plasma-sprayed ceria stabilised zirconia coatings to oxygen rich environments

Plasma-sprayed zirconia coatings are widely used for oxidation protection. Up-to-date, microstructural stabilisation of such coatings is mainly achieved with yttrium oxide; however recent scientific attempts indicated that ceria stabilised zirconia coatings could be a very promising alternative. In the present work, a coating of this kind has been deposited onto a Ni-based superalloy with the interference of a NiCrAlY bond coating by plasma spraying. Its oxidation resistance was estimated with thermogravimetric analysis with exposure at 1100 °C in air. The microstructure of the as-sprayed coating was studied with X-ray diffraction and electron microscopy before and after oxidation. From this examination it was deduced that ceria stabilised zirconia (CSZ) coating is rather stable at the temperature under question. However reduction of ceria takes place at larger exposure periods.

Sintering characteristics of plasma sprayed zirconia coatings containing different stabilisers

Zirconia powders with different types of stabiliser (Y 2O 3, Dy 2O 3 and Yb 2O 3) have been air plasma sprayed onto metallic substrates. The coatings were detached and dimensional changes during heat treatment were measured by dilatometry. Ytterbia-stabilised specimens exhibited the highest rates of shrinkage, in both in-plane and through-thickness directions. However, it was noted that these specimens had higher initial porosity levels, and a finer microstructure, than coatings containing the other stabilisers. In-plane stiffness and through-thickness thermal conductivity were also measured after different heat treatments. These increased at greater rates for specimens with higher porosity levels (ie the Yb-stabilised coatings). Changes in pore architecture during heat treatments were also studied. Fine scale porosity is rapidly reduced during heat treatment. This correlates with enhanced inter-splat bonding and healing of intra-splat microcracks. In general, the sintering behaviour, and consequent changes in microstructure and properties, appear to be more sensitive to the pore architecture than to stabiliser type. This is correlated with theoretical expectations that it is grain boundary and surface diffusion which will dominate the sintering behaviour, rather than lattice diffusion, and these are more likely to be affected by pore structure, and possibly by the presence of certain types of impurity, than by stabiliser content. It is also noted that thermal cycling appears to retard sintering, at least in terms of the rate of shrinkage. This effect, which could be of practical significance, is briefly discussed.

A stochastic coating model to predict the microstructure of plasma sprayed zirconia coatings

A stochastic model has been developed to simulate the formation of thermal sprayed yttria-stabilized zirconia coatings. Porosity in the ceramic coating was assumed to be due to incomplete filling of cavities on the rough surface of the coating. Values of thermal spray particle size, velocity, temperature and trajectories were generated assuming they followed known distributions, with means and standard deviations measured experimentally. Predictions from the model of porosity and roughness of zirconia coatings agreed with experimental results, accurately predicting a reduction in porosity with particle size. The model also predicted that porosity, thickness and roughness would decrease with increasing particle velocity. Increasing spray-gun standoff distance had little effect on porosity (keeping particle velocity constant), while it decreased coating thickness and roughness.

A Comparison between CoNiCrAlY Bond Coat and Zirconia Plasma Sprayed Coatings on Creep Tests

Thermomechanical and electrical properties of zirconia-based ceramics have led to a wide range of advanced and engineering ceramic applications like solid electrolyte in oxygen sensors, fuel cells and furnace elements and its low thermal conductivity has allowed its use for thermal barrier coatings for aerospace engine components. A comparison between CoNiCrAlY bond coat and zirconia plasma sprayed coatings on creep tests of the Ti-6Al-4V alloy was studied. The material used was commercial Ti-6Al-4V alloy. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti-6Al-4V substrates by Sulzer Metco Type 9 MB. Constant load creep tests were conducted on a standard creep machine in air on coated samples, at stress levels of 520 MPa at 500°C to evaluate the oxidation protection on creep of the Ti-6Al-4V alloy. Results indicate that the creep resistance of the ceramic coating was greater than metallic coating.

Thermophysical, mechanical and microstructural characterization of aged free-standing plasma-sprayed zirconia coatings

The effect of porosity on the thermal diffusivity and elastic modulus has been studied on artificially aged, free-standing thermal barrier coatings (TBCs) produced by air plasma spray (APS). The activation energy of the sintering phenomenon was estimated from the variation in diffusivity with time and temperature. X-ray diffraction was used to evaluate the phase stability of 7 wt.% yttria partially stabilized zirconia (YPSZ) coatings. The thermal diffusivity and elastic modulus as measured by photothermal techniques and three-point bending, respectively, are reported as a function of the ageing time. Correlations between the thermal and mechanical parameters are investigated by suitable models based on the microstructural features revealed by electron microscopy. The reliability of porosity information provided by image analysis and used as input for the modelling is critically discussed.